By Martin Burnicki with some edits by Harlan Stenn
The NTP software protocol and its reference implementation, ntpd, were originally designed to distribute UTC time over a network as accurately as possible.
Unfortunately, leap seconds are scheduled to be inserted into or deleted from the UTC time scale in irregular intervals to keep the UTC time scale synchronized with the Earth rotation. Deletions haven’t happened, yet, but insertions have happened over 30 times.
The problem is that POSIX requires 86400 seconds in a day, and there is no prescribed way to handle leap seconds in POSIX.
Whenever a leap second is to be handled ntpd either:
passes the leap second announcement down to the OS kernel (if the OS supports this) and the kernel handles the leap second automatically, or
applies the leap second correction itself.
NTP servers also pass a leap second warning flag down to their clients via the normal NTP packet exchange, so clients also become aware of an approaching leap second, and can handle the leap second appropriately.
The Problem on Unix-like Systems
If a leap second is to be inserted then in most Unix-like systems the OS kernel just steps the time back by 1 second at the beginning of the leap second, so the last second of the UTC day is repeated and thus duplicate timestamps can occur.
Unfortunately there are lots of applications which get confused it the system time is stepped back, e.g. due to a leap second insertion. Thus, many users have been looking for ways to avoid this, and tried to introduce workarounds which may work properly, or not.
So even though these Unix kernels normally can handle leap seconds, the way they do this is not optimal for applications.
One good way to handle the leap second is to use ntp_gettime() instead of the usual calls, because ntp_gettime() includes a "clock state" variable that will actually tell you if the time you are receiving is OK or not, and if it is OK, if the current second is an in-progress leap second. But even though this mechanism has been available for about 20 years' time, almost nobody uses it.
The Leap Smear Approach
Due to the reasons mentioned above some support for leap smearing has recently been implemented in ntpd. This means that to insert a leap second an NTP server adds a certain increasing "smear" offset to the real UTC time sent to its clients, so that after some predefined interval the leap second offset is compensated. The smear interval should be long enough, e.g. several hours, so that NTP clients can easily follow the clock drift caused by the smeared time.
During the period while the leap smear is being performed, ntpd will include a specially-formatted refid in time packets that contain "smeared" time. This refid is of the form 254.x.y.z, where x.y.z are 24 encoded bits of the smear value.
With this approach the time an NTP server sends to its clients still matches UTC before the leap second, up to the beginning of the smear interval, and again corresponds to UTC after the insertion of the leap second has finished, at the end of the smear interval. By examining the first byte of the refid, one can also determine if the server is offering smeared time or not.
Of course, clients which receive the "smeared" time from an NTP server don’t have to (and even must not) care about the leap second anymore. Smearing is just transparent to the clients, and the clients don’t even notice there’s a leap second.
Pros and Cons of the Smearing Approach
The disadvantages of this approach are:
During the smear interval the time provided by smearing NTP servers differs significantly from UTC, and thus from the time provided by normal, non-smearing NTP servers. The difference can be up to 1 second, depending on the smear algorithm.
Since smeared time differs from true UTC, and many applications require correct legal time (UTC), there may be legal consequences to using smeared time. Make sure you check to see if this requirement affects you.
However, for applications where it’s only important that all computers have the same time and a temporary offset of up to 1 s to UTC is acceptable, a better approach may be to slew the time in a well defined way, over a certain interval, which is what we call smearing the leap second.
The Motivation to Implement Leap Smearing
Here is some historical background for ntpd, related to smearing/slewing time.
Up to ntpd 4.2.4, if kernel support for leap seconds was either not available or was not enabled, ntpd didn’t care about the leap second at all. So if ntpd was run with -x and thus kernel support wasn’t used, ntpd saw a sudden 1 s offset after the leap second and normally would have stepped the time by -1 s a few minutes later. However, ntpd -x does not step the time but "slews" the 1-second correction, which takes 33 minutes and 20 seconds to complete. This could be considered a bug, but certainly this was only an accidental behavior.
However, as we learned in the discussion in http://bugs.ntp.org/2745, this behavior was very much appreciated since indeed the time was never stepped back, and even though the start of the slewing was somewhat undefined and depended on the poll interval. The system time was off by 1 second for several minutes before slewing even started.
In ntpd 4.2.6 some code was added which let ntpd step the time at UTC midnight to insert a leap second, if kernel support was not used. Unfortunately this also happened if ntpd was started with -x, so the folks who expected that the time was never stepped when ntpd was run with -x found this wasn’t true anymore, and again from the discussion in NTP bug 2745 we learn that there were even some folks who patched ntpd to get the 4.2.4 behavior back.
In 4.2.8 the leap second code was rewritten and some enhancements were introduced, but the resulting code still showed the behavior of 4.2.6, i.e. ntpd with -x would still step the time. This has only recently been fixed in the current ntpd stable code, but this fix is only available with a certain patch level of ntpd 4.2.8.
So a possible solution for users who were looking for a way to come over the leap second without the time being stepped could have been to check the version of ntpd installed on each of their systems. If it’s still 4.2.4 be sure to start the client ntpd with -x. If it’s 4.2.6 or 4.2.8 it won’t work anyway except if you had a patched ntpd version instead of the original version. So you’d need to upgrade to the current -stable code to be able to run ntpd with -x and get the desired result, so you’d still have the requirement to check/update/configure every single machine in your network that runs ntpd.
Google’s leap smear approach is a very efficient solution for this, for sites that do not require correct timestamps for legal purposes. You just have to take care that your NTP servers support leap smearing and configure those few servers accordingly. If the smear interval is long enough so that NTP clients can follow the smeared time it doesn’t matter at all which version of ntpd is installed on a client machine, it just works, and it even works around kernel bugs due to the leap second.
Since all clients follow the same smeared time the time difference between the clients during the smear interval is as small as possible, compared to the -x approach. The current leap second code in ntpd determines the point in system time when the leap second is to be inserted, and given a particular smear interval it’s easy to determine the start point of the smearing, and the smearing is finished when the leap second ends, i.e. the next UTC day begins.
The maximum error doesn’t exceed what you’d get with the old smearing caused by -x in ntpd 4.2.4, so if users could accept the old behavior they would even accept the smearing at the server side.
In order to affect the local timekeeping as little as possible the leap smear support currently implemented in ntpd does not affect the internal system time at all. Only the timestamps and refid in outgoing reply packets to clients are modified by the smear offset, so this makes sure the basic functionality of ntpd is not accidentally broken. Also peer packets exchanged with other NTP servers are based on the real UTC system time and the normal refid, as usual.
The leap smear implementation is optionally available in ntp-4.2.8p3 and later, and the changes can be tracked via http://bugs.ntp.org/2855.
Using NTP’s Leap Second Smearing
Leap Second Smearing MUST NOT be used for public servers, e.g. servers provided by metrology institutes, or servers participating in the NTP pool project. There would be a high risk that NTP clients get the time from a mixture of smearing and non-smearing NTP servers which could result in undefined client behavior. Instead, leap second smearing should only be configured on time servers providing dedicated clients with time, if all those clients can accept smeared time.
Leap Second Smearing is NOT configured by default. The only way to get this behavior is to invoke the ./waf configure script from the NTP source code package with the --enable-leap-smear parameter before the executables are built.
Even if ntpd has been compiled to enable leap smearing support, leap smearing is only done if explicitly configured.
The leap smear interval should be at least several hours' long, and up to 1 day (86400s). If the interval is too short then the applied smear offset is applied too quickly for clients to follow. 86400s (1 day) is a good choice.
If several NTP servers are set up for leap smearing then the same smear interval should be configured on each server.
Smearing NTP servers DO NOT send a leap second warning flag to client time requests. Since the leap second is applied gradually the clients don’t even notice there’s a leap second being inserted, and thus there will be no log message or similar related to the leap second be visible on the clients.
Since clients don’t (and must not) become aware of the leap second at all, clients getting the time from a smearing NTP server MUST NOT be configured to use a leap second file. If they had a leap second file they would apply the leap second twice: the smeared one from the server, plus another one inserted by themselves due to the leap second file. As a result, the additional correction would soon be detected and corrected/adjusted.
Clients MUST NOT be configured to poll both smearing and non-smearing NTP servers at the same time. During the smear interval they would get different times from different servers and wouldn’t know which server(s) to accept.
Setting Up A Smearing NTP Server
If an NTP server should perform leap smearing then the leap smear interval (in seconds) needs to be specified in the NTP configuration file ntp.conf, e.g.:
Please keep in mind the leap smear interval should be between several and 24 hours' long. With shorter values clients may not be able to follow the drift caused by the smeared time, and with longer values the discrepancy between system time and UTC will cause more problems when reconciling timestamp differences.
When ntpd starts and a smear interval has been specified then a log message is generated, e.g.:
ntpd: config: leap smear interval 86400 s
While ntpd is running with a leap smear interval specified the command:
ntpq -c rv
reports the smear status, e.g.:
# ntpq -c rv associd=0 status=4419 leap_add_sec, sync_uhf_radio, 1 event, leap_armed, version="ntpd 4.2.8p3-RC1@1.3349-o Mon Jun 22 14:24:09 UTC 2015 (26)", processor="i586", system="Linux/3.7.1", leap=01, stratum=1, precision=-18, rootdelay=0.000, rootdisp=1.075, refid=MRS, reftime=d93dab96.09666671 Tue, Jun 30 2015 23:58:14.036, clock=d93dab9b.3386a8d5 Tue, Jun 30 2015 23:58:19.201, peer=2335, tc=3, mintc=3, offset=-0.097015, frequency=44.627, sys_jitter=0.003815, clk_jitter=0.451, clk_wander=0.035, tai=35, leapsec=201507010000, expire=201512280000, leapsmearinterval=86400, leapsmearoffset=-932.087
In the example above leapsmearinterval reports the configured leap smear interval all the time, while the leapsmearoffset value is 0 outside the interval and increases from 0 to -1000 ms over the interval. So this can be used to monitor if and how the time sent to clients is smeared. With a leapsmearoffset of -.932087, the refid reported in smeared packets would be 254.196.88.176.